Research groups
  • Stimuli-responsive hydrogels

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Stimuli-responsive associative polymers and hydrogels

transitionAssociative polymers are of industrial importance owing to their adjustable rheological properties for example in paints, cosmetics formulations or pharmaceutical as well as medical applications. Particularly interesting are the stimuli-responsive systems of associating polymers in which free standing gels transform to free-flowing liquids or vice versa. To obtain such systems water-soluble polymers modified with a small number of hydrophobic groups are often used. Enhanced viscosity and reversible gelling behaviour originate from transient intermolecular associations between the hydrophobic groups.

Starlike polyelectrolytes, four-armed poly(acrylic acid)-block-polystyrene (PS-b-PAA)4 with central hydrophobic PS core and hydrophilic PAA blocks1 or with reverse block order2,3 ((PAA-b-PS)4)  were prepared via atom transfer radical polymerization (ATRP). These water-soluble polymers associate in aqueous solutions via the interaction of the PS blocks. The effects of polymer structure and concentration as well as external conditions (temperature, ionic strength, pH) for these systems has been investigated.1,2,3

PNIPAMSynthesis and solution behaviour of stereoblock polymers of poly(N-isopropyl acrylamide) PNIPAM made via reversible addition–fragmentation chain transfer polymerization (RAFT) have been studied.4-8  Atactic PNIPAM is a thermosensitive water soluble polymer that dissolves in water at low temperatures but loses it's solubility upon heating at about 31 °C. Other stereoregular forms of PNIPAM have different solubilities, syndiotactic PNIPAM being poorly water soluble and isotactic form insoluble in water. So called stereoregular block polymers comprising of isotactic (i) and atactic (a) PNIPAM sequences were studied, the polymers having either i-a-i or a-i-a block sequence. Increasing the lengths of the isotactic blocks makes polymers less water soluble, so that some of the polymers dissolve only upon cooling to 5°C. At room temperature the isotactic blocks associate to form different type of aggregates, and, for i-a-i type polymers at higher concentrations, form hydrogels while still retaining the temperature sensitive properties. It has been shown that in acetone-water mixtures these polymers are capable of controlling the growth of drug crystals.6,7


1. Supramolecular Assemblies of Amphiphilic PMMA-block-PAA Stars in Aqueous Solutions
S. Strandman , S. Hietala, V. Aseyev, B. Koli , S. J. Butcher, H. Tenhu
Polymer, 47 (2006) 6524-6535.

2. Synthesis and Rheological Properties of an Associative Star Polymer in Aqueous Solutions.
S. Hietala, P. Mononen, S. Strandman, P. Järvi, M. Torkkeli,  K. Jankova, S. Hvilsted, H. Tenhu Polymer, 48 (2007) 4087-4096.

3. Rheological Properties Of Associative Star Polymers In Aqueous Solutions - Effect Of Hydrophobe Length And Polymer Topology.
S. Hietala, S. Strandman, P. Järvi, M. Torkkeli,  K. Jankova, S. Hvilsted, H. Tenhu
Macromolecules, 42 (2009) 1726-1732.

4. A-B-A Stereoblock Copolymers of N-isopropylacrylamide
M. Nuopponen, K. Kalliomäki, A. Laukkanen, S. Hietala, H. Tenhu
J. Polym. Sci, Polym. Chem., 46 (2008) 38-46.

5. Thermoassociating Poly(N-Isopropylacrylamide) A-B-A Stereoblock Copolymers
S. Hietala,M. Nuopponen, K. Kalliomäki, H. Tenhu Macromolecules, 41 (2008) 2627 - 2631.

6. Behaviour of stereoblock poly(N-isopropyl acrylamide) in acetone–water mixtures
T. Munk, S. Hietala, K. Kalliomäki, M. Nuopponen, H. Tenhu, F. Tian, J. Rantanen, S. Baldursdottir
Polym. Bull., 46 (2011) 677-692

7. Crystal Morphology Modification by the Addition of Tailor-Made Stereocontrolled Poly(N-isopropyl acrylamide)
T. Munk, S. Baldursdottir, S. Hietala, T. Rades, S. Kapp, M. Nuopponen, K. Kalliomaki, H. Tenhu, J. Rantanen
Mol. Pharm. 9 (2012) 1932–1941.

8. Interfacial and fluorescence studies on stereoblock poly(N-isopropylacryl amide)s
J. Niskanen, C. Wu, M. Bynum, G. Fuller, H. Tenhu, S. Hietala
Langmuir, 28 (2012) 14792–14798.